OFDM is used as a transmission technique for digital terrestrial broadcasting in Europe and Japan, wireless LAN, and the like. The OFDM technique is a method of performing modulation/demodulation by assigning data to a plurality of carriers orthogonal to each other. An Inverse Fast Fourier Transform (IFFT) process is performed in a transmitter, while a Fast Fourier Transform (FFT) process is performed in a receiver. Any modulation method can be applied to each carrier, including QPSK (Quaternary Phase Shift Keying), QAM (Quadrature Amplitude Modulation), and the like.
Generally, in apparatuses receiving digital terrestrial broadcasting, frequency characteristics of a transmission channel (channel response) is estimated based on a pilot signal inserted in a received OFDM signal, and the estimated channel response is used to perform waveform equalization.
An exemplary technique concerning such channel response estimation and waveform equalization is described in Patent Document 1 below. In Patent Document 1, a received pilot signal is separated from an OFDM signal which has been converted into a frequency domain signal by an FFT circuit, and is divided by a known pilot signal to obtain the channel response of a carrier which transmits the received pilot signal. Further, the channel response is smoothed in a time direction by a symbol filter, i.e., interpolation is performed between symbols, and thereafter, interpolation is performed between carriers by an interpolation circuit, to obtain a channel response H(l, kd). A received data signal Y(l, kd) is divided by the channel response H(l, kd) to obtain equalized data X(l, kd).
When a channel response is estimated by performing interpolation between symbols and between carriers using a filter in this manner, noise overlaps the estimated channel response. A power of the noise (noise power) increases with an increase in pass band width of a filter for interpolation between carriers, and decreases with a decrease in the pass band width. Therefore, in order to remove an influence of the noise to improve the precision of channel estimation, it is effective to narrow the pass band width of the filter.
According to the transmission standards for digital terrestrial television broadcasting in Europe and Japan, any one of Tu/4, Tu/8, Tu/16, and Tu/32 (Tu: effective symbol duration) is adopted as a guard interval so as to transmit an OFDM signal. In a receiver, a delay time of multipath interference which can be removed is determined, depending on the guard interval. For example, when the guard interval is Tu/4, a main wave can be received while removing an influence of a delayed wave having a delay time of Tu/4 or less. Therefore, in order to remove the multipath influence to improve channel estimation precision, it is effective to widen the pass band width of the filter.
Therefore, in the interpolation circuit described in Patent Document 1, a guard interval is determined from a received OFDM signal, and based on the determined guard interval, a coefficient of a filter which is used to perform interpolation between carriers (hereinafter referred to as a “carrier filter”) in the interpolation circuit is controlled so that pass band widths of the filter are switched. Specifically, when the guard interval is long, the filter coefficient is set so as to widen the pass band width. When the guard interval is short, the filter coefficient is set so as to narrow the pass band width.
As described above, in the interpolation circuit, the band widths of the filter for interpolation are switched so as to fit the guard interval, thereby suppressing and reducing a noise component overlapping the estimated channel response.
Patent Document 1: Japanese Patent Unexamined Publication No. 11-163822